The present invention relates to a sieve belt comprised of a multiplicity of helices made of thermosettable synthetic resin material, especially synthetic resin wire, with adjacent helices intermeshed with each other so that the windings of one helix enter between the windings of the adjacent helix and pintle wires which are inserted through the respective channels thus formed by the intermeshed helices. For controlling the air permeability of the sieve belt the hollow interiors of the helices are filled with a filler material. The invention further relates to a method for producing such a sieve belt.
Due to varying requirements, it is desirable to be able to change the air permeability of sieve belts made of synthetic resin helices. In the sieve belt disclosed in U.S. patent application Ser. No. 111,497 filed Jan. 11, 1980 now U.S. Pat No. 4,346,138 in the name of Johannes Lefferts and assigned to the same assignee as the present application, the spirals or helices are open and the air permeability is very high. In papermaking machines operating at very high speeds, high air permeability may be disadvantageous since it causes very intense air circulation which may disturb the paper web. The air permeability could be reduced by inserting stiff monofilaments into the interiors of the helices from the sieve belt edges or by inserting spun yarns or multifilament yarns by means of a threading device. However, such inserted material would lie straight in the interiors of the helices so that a large amount of filling material would be required to appreciably reduce the air permeability. Moreover, the large amount of filler material would greatly increase the weight per unit area of the sieve so that the insertion of the filler material and generally the handling of the sieve would become cumbersome, especially in the mounting of the sieve belt on the papermaking machine. The later introduction of filler material into the assembled sieve belt meets with difficulties and brings about disadvantages. Either the filler materials are introduced into the interlocked helices before the sieve belt is thermoset or the filler materials are inserted into and threaded through the channels after thermosetting. In both cases, the sieve belt must be thermoset a second time after insertion of the filler material since otherwise, the filler material might shrink later on under the influence of the papermachine temperature. Two thermosetting steps are very expensive and time consuming. Moreover, when the filler material is introduced prior to thermosetting of the sieve belt, there is the risk that the helices may shift over the pintle wires which are still straight at that stage so that humps and buckles may develop in the sieve belt. Furthermore, in both modes of operation, a certain length of filler material would have to extend laterally from the sieve belt so that after thermosetting and shrinkage of the filler material, the sieve belt will still be filled across its entire width. Such a method would be complicated and susceptible to trouble.
Another disadvantage resides in the fact that the filler material extends straight through the helices so that it can easily slip out of the sieve belt. For instances, if the edge of the sieve belt is damaged in the papermaking machine, the filler material can easily get caught on parts of the papermaking machine and will then be pulled out of the sieve belt. This may happen when the sieve belt laterally chafes against the machine.